1. Field of the Invention
The present invention relates to a method and a device for controlling electrical switchgear. More particularly, the invention relates to a method and a device that continuously and automatically optimizes switchgear performance.
2. Description of Related Art
In a power distribution system, switchgear are typically employed to protect the system against abnormal conditions. Abnormal conditions include, for example, power line fault conditions or irregular loading conditions. In general, switchgear are well-known in the art.
There are different types of switchgear for different applications. A fault interrupter is one type of switchgear. Fault interrupters are employed for automatically opening a power line upon the detection of a fault condition. Reclosers are another type of switchgear. In response to a fault condition, reclosers, unlike fault interrupters, rapidly trip open and then reclose the power line a number of times in accordance with a set of time-current curves. Then, after a pre-determined number of trip/reclose operations, the recloser will "lock-out" the power line if the fault condition has not been cleared. A breaker is a third type of switchgear. Breakers are similar to reclosers; however, they are generally capable of performing only an open-close-open sequence, and their interruption ratings are significantly higher than reclosers. A capacitor switch is a fourth type of switchgear. Capacitor switches are used for energizing and de-energizing capacitor banks. Capacitor banks are used for regulating the line current feeding large loads (e.g., industrial loads) when the load causes the line current to lag behind the line voltage. Upon activation, a capacitor bank pushes the line current back into phase with the line voltage, thereby boosting the power factor (i.e., the amount of power being delivered to the load). Capacitor switches generally perform one open or one close operation at a time.
As the switchgear contacts come into proximity with one another (i.e., during a closing operation) or when the contacts first separate (i.e., during an opening operation), some amount of arcing occurs between the contacts. Arcing can cause an excessive amount of heat to build up on the surface of the contacts, and accordingly, cause the contacts to wear-out at an excessively fast rate. Arcing can also strain or damage system components such as power transformers. Therefore, arcing is highly undesirable.
In general, all switchgear, irrespective of switchgear type, attempt to minimize arcing. Some switchgear designs attempt to accomplish this by driving the switchgear contacts apart (i.e., during an opening operation) or together (i.e., during a closing operation) as fast as possible. The theory behind this method is that if the amount of time the contacts spend in close proximity to one another is minimized, arcing is also minimized. In practice, this strategy is flawed, particularly during closing operations, because the contacts tend to bounce when they come into physical contact with each other as the relative velocity of the contacts increases. Contact bounce, in turn, leads to the generation of undesirable transient voltage and current events.
A more effective method for minimizing arcing and minimizing the generation of transients is to synchronize the initiation of the switchgear operation so that the actual closing or opening of the contacts occurs when the AC voltage or current across the contacts is at zero volts or zero amperes, respectively. For example, in FIG. 1, it is preferable that a closing of the contacts occurs when the AC voltage waveform 100 passes through a zero-voltage crossover point, such as point A. Generally, it is preferable to close at a voltage zero across the switchgear contacts and to open at a current zero to minimize arc time. Normal arc interruptions occur at a current zero. For a capacitor switch application, the capacitor load current leads the voltage by 90 electrical degrees. Therefore, the current waveform does not need to be monitored and it can be assumed that at a voltage zero the current is at a peak and at a current zero the voltage is at a peak. For true synchronous operations for other applications, both the voltage waveform and current waveform need to be monitored.
Present switchgear designs that employ a synchronizing method generally do so by pre-defining a fixed amount of time t.sub.1, where t.sub.1 is equal to a presumed AC voltage waveform period T less an amount of time t.sub.2, wherein t.sub.2, in turn, is an approximate amount of time required to complete the switchgear operation. For example, in FIG. 1, if the AC voltage waveform is operating at 60 Hz, the period T of the AC waveform 100 is 16.66 msec. If the pre-defined time t.sub.2 is 11.66 msecs, then t.sub.1 is 5 msecs. Accordingly, if a switchgear employing this method receives a command to initiate a close operation, the switchgear will detect a next zero-voltage crossover point, such as crossover point B in FIG. 1, then wait t.sub.1 msecs, which corresponds with point C in FIG. 1, to initiate the switching operation. Likewise, if an open command is received, the switchgear will detect a next zero current crossover point and determine an appropriate opening point that is somewhat similar to the timing sequence described above for the closing operation. The opening point is determined such that at the next zero current crossover a sufficient contact opening gap is established that will interrupt the flow of current and withstand the power system recovery voltage to prevent reignitions or restrikes. From here on, the discussion will focus on synchronized voltage switching. However, it will be understood by one skilled in the art that switching could also be synchronized with the current waveform on opening.
Unfortunately, the fixed time synchronization method does not always produce accurate results. First, the AC voltage waveform 100 rarely propagates at exactly 60 Hz. In fact, it generally fluctuates slightly above and below 60 Hz. Accordingly, the period T of the AC voltage waveform 100 will fluctuate. Therefore, initiating a switching operation at point C does not always guarantee a synchronized opening or closing operation (i.e., an operation that is synchronized with a zero-voltage crossover point). Second, conditions such as ambient temperature can affect the dynamic friction of the mechanism and change the actual amount of time that it takes for the contacts to complete the switching operation. Therefore, the amount of time represented by t.sub.2 may fluctuate with temperature. Thus, once again, initiating the switching operation at point C is not likely to consistently result in a synchronized opening or closing operation. Third, over the life of the switchgear, the distance the contacts must travel during a switching operation generally increases. This is due to ordinary contact wear and wear from the components of the mechanism. As the contact travel distance increases, it becomes less and less likely that initiating the switching operation at point C as a function t.sub.1, t.sub.2 and T will result in a synchronized switching operation.
In the particular case of a capacitor switch, minimizing arcing and minimizing the generation of transients is especially important. That is because even small inaccuracies in synchronizing a switching operation with a zero-voltage crossover point on the AC voltage waveform can result in arcing and/or transients that involve thousands of amperes and volts. Therefore, an enormous demand exists for a switchgear design, particularly a capacitor switch design, that provides more accurate, point-on-wave switching operation control, to better assure zero-voltage switching operations to minimize transient effects.